The invention involves a vertical-axis air classifier with central product feed, with a tangential classifying air supply inlet located on a level with the rotor, with a stationary guide vane ring arranged at a radial distance around the periphery of the rotor, with an annular-shaped classifying zone delimited by a deflector-wheel rotor with one-sided bearing and a vane guide ring located coaxially at a radial distance to the outer periphery of the rotor, with a drive shaft for the rotor with one-sided bearing as well as a housing with fine material and coarse material discharge.
An air classifier of this design, where the material to be classified is set into rotary motion by a wheel which rotates around an axis and the classifying air flows centripetally to distribute the rotating material uniformly over the periphery of the classifying wheel to the classifying zone, is already known from the German patent application DE 35 21 638 A1.
The material to be classified is charged centrally to the classifier, is then distributed over a large surface area by a centrifugal plate and routed as a bell-shaped cloud of product over the periphery of the rotor past the classifying wheel vanes. The classifying air flows through the classifying wheel in centripetal direction; the fines are routed to the inside of the rotor. Gravity causes the rejected coarse material to move downwards, where it deposits in an annular-shaped coarse material discharge chamber.
The air flow pattern through the classifying zone is centripetal. The rotating deflector wheel deflects the coarse material radially to the outside and conveys the fines together with the classifying air to the inside of the rotor. The classified fines are then deflected axially downwards and finally discharged from the rotor.
The drive and bearing of the rotor are arranged above the rotor on the same side as the central product feed.
The specification of achieving a steady and continuous product feed is resolved fairly well here by the central product feed. Because the area above the centrifugal plate of the rotor must be kept clear for the central product feed, this design has the drive and bearing located in the same zone. The current state of the art permits only an unsatisfactory arrangement of central product feed, drive and bearing with the aid of a hollow drive shaft. This design, especially that of the bearing, is particularly complex and only suitable for low speeds and moreover extremely difficult to install.
A further disadvantage of this classifier is the discharge of the coarse material, which is executed with the aid of a feed channel which runs downwards at an angle. The task of this feed channel is to route the coarse material which enters and distributes throughout the entire coarse material discharge chamber to a discharge socket located at only one point around the periphery of the coarse material discharge chamber.
For the classifier designed in accordance with this state of the art, the problem of the coarse material discharge has been satisfactorily resolved. Disadvantageous, however, is that the axial feed channel must be large in dimension.
Another state-of-the-art design with central product feed from above is described in the German patent DE-PS 894 803. The main feature of this air classifier is the one-sided rotor bearing with a fines discharge located on the drive side. The feed material is fed centrally above the closed cover plate of the rotor. With the vertical arrangement of rotor and drive shaft, the cover plate acts as a centrifugal plate and distributes the feed material uniformly across the entire extent of the rotor. This results in a uniformly distributed cloud of product which flows over the periphery of the rotor.
The classifying air flows through the rotor centripetally and transports the fines to the inside of the rotor, whereas the coarse material is rejected by the rotating vanes. The classified fines are ultimately discharged from the rotor in centrifugal direction.
In the case of the air classifier built in accordance with this state of the art, the cover plate is supported together with the classifying wheel vanes by an extended section of the drive shaft which penetrates the rotor. The disadvantage here is that the inside of the rotor is thus not completely free from fittings and this hinders an ideal flow pattern in the inside of the rotor.
Another disadvantage is that the fines discharge is not reliably sealed off from the classifying zone, so that spatter grain can contaminate the already classified fines by entering through the gap between the classifying wheel vanes and the housing.
The design is not very stable altogether and removal plus installation of the rotor in the housing is extremely difficult. Such an air classifier is particularly unsuitable for high speeds.
Air classifiers are known, for example, from the German patent DE-PS 36 38 915 C 2--which besides a rotor that is driven on one side also have a fines discharge that leads axially downwards.
However, the coarse material discharge is still arranged on the same plane as the classifying wheel vanes and leads to the outside in radial direction. The assembly effort with this design is fairly high and cleaning is extremely time-consuming.
With the newer classifiers, especially those used for high-tech products, great store is laid by the ease of dismantling the entire classifier and by the ease of cleaning. This is particularly important for pharmaceutical products, pigments, ultrafine powders and toner.
With the above-mentioned materials, the batches tend to be on the small side and the product is changed frequently. It is therefore obvious that the ease of cleaning and dismantling is a major requirement.
This requirement demands a compact and space-saving classifier design where the zones such as classifying air supply, fines discharge and coarse material discharge are positioned close to each other.
To permit thorough cleaning of the inside of the classifier, it must be completely dismantled into its component parts. But because of the fact that the functional connections such as drive, bearing, product feed, fines discharge and coarse material discharge are scattered around the circumference of the classifier, this is particularly difficult.
With state-of-the-art classifier housings, the cleaning apertures are window-like openings located usually on the side of the classifier housing. The horizontal removal of product deposits from these lateral apertures is, however, extremely difficult because any product deposits dislodged from the inside walls of the classifier housing tend to fall vertically downwards as a result of the force of gravity, and not through the horizontally positioned cleaning apertures.
In the case of a classifier of this design with a relatively short axial extent of the classifying wheel and a product circulation system whereby the product is only presented to the classifying wheel once, it is disadvantageous if the residence time of the product in the classifying zone is too short. The quality of the coarse material suffers because it is conceivable that not all the fines are separated out of the material and thus the fines portion in the discharged coarse material is still relatively high.
With classifiers of this type, the material to be classified is routed from top to bottom along the rotor by the effects of gravity. Except for the radial turbulence, the feed material essentially follows the path of gravity downwards. This means that the feed material is only presented to the classifier once and then only for a brief period. Because of the short residence time of the material in the classifier, the fines cannot be diverted out of the material stream and classified in their entirety. The rejected coarse material fraction thus still contains a high proportion of fine particles. As a result of this, the coarse material quality falls short of optimal.
In addition, the vertical arrangement of the classifier and the resultant downwards flow direction of the feed material leads to a fluctuating concentration of material in the classifying zone, which in turn means that it is not possible to optimally utilize the entire length of the rotor.
Because with conventional classifiers, a mixture of fresh feed material and already classified product occurs in the outside classifying zone, the residence time of individual particles varies greatly with negative consequences for the quality of classification. The residence time, however, is an important parameter for the classification quality. It dictates the number of times the particles make contact with the classifying wheel or the guide vanes and thus the probability of the fine particles being dislodged from the coarse particles.
Another problem with conventional classifiers is the tendency of the fluid-particle suspension to separate into a force field. This can lead locally to particle concentrations which are too high. And the more concentrated the particles, the harder it is to separate the fines from the material being classified. This also has an adverse effect on the quality of classification.